1. Field of the Invention
The present invention relates to a catalyst for the oxidation of ethylene to ethylene oxide consisting of a critical combination of silver, alkali metal such as cesium, boron and sulfur deposited on a support such as alpha alumina and to the production of ethylene oxide using the catalyst; a fluorine or chloride component optionally can be included. The catalyst is essentially free of rhenium or transition metal components.
2. Description of the Prior Art
Processes for the production of ethylene oxide involve the vapor phase oxidation of ethylene with molecular oxygen using a solid catalyst comprised of silver on a support such as alumina. There have been efforts by many workers to improve the effectiveness and efficiency of the silver catalyst for producing ethylene oxide. U.S. Pat. No. 5,051,395 provides a comprehensive analysis of these efforts of prior workers.
Among the many prior teachings in this area is that of U.S. Pat. No. 4,007,135 (see also UK 1,491,447) which teaches variously silver catalysts for the production of ethylene and propylene oxides comprised of a promoting amount of copper, gold, magnesium, zinc, cadmium, mercury, strontium, calcium, niobium, tantalum, molybdenum, tungsten, chromium, vanadium, and/or preferably barium, in excess of any present in immobile form in the performed support as impurities or cements (column 2, lines 1-15), silver catalysts for the production of propylene oxide comprising a promoting amount of at least one promoter selected from lithium, potassium, sodium, rubidium, cesium, copper, gold, magnesium, zinc, cadmium, strontium, calcium, niobium, tantalum, molybdenum, tungsten, chromium, vanadium and barium, in excess of any present in immobile form in the preformed support as impurities or cements (column 2, lines 16-34), as well as silver catalysts for producing ethylene oxide or propylene oxide comprising (a) a promoting amount of sodium, cesium, rubidium, and/or potassium, and (b) magnesium, strontium, calcium and/or preferably barium in a promoting amount (column 3, lines 5-8).
U.S. Pat. No. 5,057,481, and related U.S. Pat. No. 4,908,343 are concerned with silver ethylene oxide catalysts comprised of cesium and an oxyanion of a group 3b to 7b element.
U.S. Pat. No. 3,888,889 describes catalysts suitable for the oxidation of propylene to propylene oxide comprised of elemental silver modified by a compound of an element from Group 5b and 6b. Although the use of supports is mentioned, there are no examples. The use of cesium is not mentioned.
European Publication 0 266 015 deals with supported silver catalysts promoted with rhenium and a long list of possible copromoters.
U.S. Pat. No. 5,102,848 deals with catalysts suitable for the production of ethylene oxide comprising a silver impregnated support also having thereon at least one cation promoter such as cesium, and a promoter comprising (i) sulfate anion, (ii) fluoride anion, and (iii) oxyanion of an element of Group 3b to 6b inclusive of the Periodic Table. Possibly for purposes of comparison since it is outside the scope of catalyst claimed, the patent shows at columns 21 and 22 a catalyst No. 6 comprised of Ag/Cs/S/F on a support.
U.S. Pat. No. 5,486,628 describes a silver catalyst promoted with alkali metal, rhenium and a rare earth or lanthanide component.
U.S. Pat. No. 5,011,807 is concerned with an ethylene oxide catalyst comprised of silver, alkali metal, a transition metal, and sulfur on alumina support. The support of choice in the preparation of commercial silver ethylene oxide catalysts has been a solid inorganic material such as alumina and silica, or titania based compounds, or combinations thereof. Alpha alumina, which may contain silica, has been an especially preferred carrier. Various patents have focused on the pretreatment of such carriers to improve the utility thereof U.S. Pat. No. 5,102,648, for example, shows repeated alpha alumina support washing with 90° C. de-ionized water prior to deposition of the catalyst components. In the same patent, the carrier was also washed with HF solution at 25° C. Later U.S. Pat. No. 6,103,916 similarly shows washing alpha alumina support with 90° C. water repeatedly prior to deposition of the catalytic components in ethylene oxide catalyst preparation.
The prior art has also disclosed that the catalyst's performance is improved with the addition of, specifically, thallium borate, JP-57-21937 (laid open on Feb. 4, 1982). Also, JP-56-105750 (laid open on Aug. 22, 1981) disclosed that the catalyst improvement is attained by adding boron along with Mo or W. In both cases, JP-57-21937 and 56-105750, the disclosures claimed that the carrier should not have more than 0.07% Na. This is in contrast with G.B. Patent 1,571,123 (1980), which has disclosed the advantage of adding both sodium and boron to the silver-ethylene oxide catalyst.
In another group of patents, boron was disclosed as a Re co-promoter. In this group it was clearly stated “ . . . it is preferable that if the catalyst contains rhenium, the catalyst also contains a rhenium co-promoter. When a co-promoter is utilized, it is selected from the group consisting of sulphur, molybdenum, tungsten, chromium, phosphorous, boron, and mixtures thereof”. Examples of this group of patents are: U.S. Pat. No. 5,545,603, U.S. Pat. No. 5,663,385, U.S. Pat. No. 5,739,075, U.S. Pat. No. 5,801,259, U.S. Pat. No. 5,929,259, U.S. Pat. No. 6,372,925, U.S. Pat. No. 6,368,998, EP 0 900 128B1, EP 0 874 688B1m EP 0716884B1, PCT/EP97/01622, and PCT/EP97/02236.
In the process of preparing the alpha-alumina carrier, it is customary to add boric acid, or its salts, as a flux agent, to the powdered alumina. This is clearly illustrated by W. Wingery, et al in, Introduction to Ceramics, 2ed edition, 1976 P 8. and M. Bengisu in Engineering Ceramics, 2001, P 157. Addition of boric acid, or its salts, to the powdered alumina before firing the carrier was also disclosed in U.S. Pat. Nos. 5,100,824, 5,145,824, 5,384,302, and WO 97/40933.
In the context of the bewildering and vast number of references, many of them contradictory, applicant has discovered a novel and improved catalyst for the production of ethylene oxide.